Removing backup for iPhone photos from iCloud

I have used Dropbox for years. It's inexpensive and user friendly and I've been backing up my photos from my iPhone on it. However, I didn't even think of them backing up to iCloud as well, and I want to stop that so I don't have to add more storage as it's filling up. More than half of what's in it is from my photos, which as I said, are automatically backed up at Dropbox. Is there a simple way to stop backing up photos in iCloud?